In particular, the invention concerns aircraft possessing full or partial vertical lift capability in which a ducted gas stream is guided by means of a vectorable nozzle. A number of aircraft having this type of system have been built or proposed. Most notable of these is the British Aerospace HARRIER powered by a Rolls-Royce PEGASUS engine which has four swivellable nozzles to vector hot and cold engine gas streams to generate thrust vectorable between vertical and horizontal directions. The engine exhaust streams are permanently ducted through the four nozzles which contain fixed guide vanes and are disposed in pairs on opposite sides of the aircraft fuselage. A penalty of the nozzle arrangement is a relatively high level of drag since they protrude permanently into airflow over the fuselage.
A development of the vectorable gas stream concept has the main propulsion engine driving one or more lift fans which exhaust selectively through stowable nozzles. These nozzles are deployed when vectored thrust is required but are otherwise stowed behind covers to avoid parasitic drag during a normal flight mode. One of the nozzle designs for this concept has retractable nozzle ducting terminated by a cascade of movable parallel vanes which are turned each about its own longitudinal axis to achieve the vectoring range. A drawback with this type of arrangement remains aerodynamic drag caused by a deployed nozzle. In the proposed arrangement the nozzle is deployed to its fullest extent and the vanes are turned to achieve downwardly directed vertical thrust and to vector the thrust rearwardly in transition to horizontal flight. As a consequence drag forces created by the nozzle increase as forward speed builds-up until such times as the gas stream supply to the nozzle can be terminated and the nozzle retracted. The present invention has for one of its objectives to avoid this drag. For another object it seeks to reduce the weight of the nozzle by avoiding the use of vectorable guide vanes.